Project Summary: The goal of this project is to determine the effects of mild traumatic brain injury (mTBI) and blast exposure on the vestibular system and CNS. Dizziness and balance disorders are common symptoms associated with mTBI or head injury. Numerous studies have provided significant evidence that mTBI or head injury can cause damage to the vestibular system; however, most have limited the vestibular evaluation to assessment of horizontal semicircular canal function. Recently, methods have been developed to assess otolith function, and there is some evidence that head injury may affect the otolith organs to a greater degree than the semicircular canals. mTBI has been called the signature condition of veterans returning from Operation Enduring Freedom/Operation Iraqi Freedom (OEF/OIF), and the cause is often related to blast exposure from improvised explosive devices, mortars or rocket-propelled grenades. Some investigators have presumed that dizziness and balance disorders following blast exposure are related to CNS damage caused by the TBI rather than the pressure wave from the blast injury. Thus, most research has focused on the vestibular consequences of TBI (or head injury), and there is limited data on the effects of blast exposure on vestibular function or balance. Recently, magnetic resonance imaging techniques have been developed that may allow for testing the assumption that the symptoms of dizziness or imbalance related to head injury or blast exposure are often due to central vestibular or CNS involvement. Specific aims of this project are to determine the effect of mTBI and blast exposure on (1) peripheral vestibular system function (specifically, horizontal semicircular canal function, and otolith organ function), (2) central vestibular/CNS function, (3) postural stability, and (4) dizziness-related quality of life. Four subject groups will include veterans complaining of dizziness/imbalance with (1) a history of blast exposure, (2) with mTBI, (3) with blast exposure and mTBI, and (4) a control group. Each subject will undergo tests of horizontal semicircular canal function (caloric and rotary chair), tests of otolith function (vestibular evoked myogenic potentials, subjective visual vertical), central vestibular function/CNS function (ocular motor tests, diffusion tensor and susceptibility weighting imaging), gait and balance testing, and the Dizziness Handicap Inventory.